ADC with digital error correction

ABSTRACT

Interleaved Analogue to Digital converter, comprising a plurality of individual ADCs ( 71 ). A digital filter stage ( 20 ) is used for equalizing the responses of the individual ADCs, and comprises a FIR filter in which the coefficient table is cyclically reloaded between ADCs&#39; samples, in order to reduce the number of multipliers required.

REFERENCE DATA

This application claims priority from European Patent Application No.EP03104679.0 filed on Dec. 12, 2003, the contents whereof are herebyincorporated by reference.

FIELD OF THE INVENTION

The present patent application relates to an Analogue to DigitalConverter (ADC), and in particular to an ADC including a plurality oftime-interleaved converters for increasing the overall conversion rateand a correction scheme for compensating the errors which might arise bymismatches in individual converters.

DESCRIPTION OF RELATED ART

In data acquisition systems using Analogue to Digital Converters (ADC),a technique of time-interleaving several identical ADC of lowerconversion rate is often used to increase the overall conversion rate.By employing a large number of individual slow ADCs, it is possible toachieve extremely high overall conversion speeds.

However, this technique finds a limitation in the fact that theindividual responses of the individual ADCs, like for example Gain,Delay, Nonlinearity or Offset, can never be strictly identical. Thismismatch is reflected in unwanted distortions of the acquired signal,which manifest themselves in the frequency domain as spurious spectralcomponents.

The problem of spurious components in interleaved digitizers is wellknown and a number of devices have been devised in order to correct orcompensate these unwanted components. On the other hand, the complexfrequency dependence of the mismatches requires complex correctionarchitectures which, in many cases, constitute a major part of theelectronic circuitry.

U.S. Pat. No. 567,030 describes a high-bandwidth interleaved acquisitionsystem in which the outputs of individual ADC are treated by digitalsynthesizers in order to compensate for individual variations of theADC. Each synthesizer may be implemented as a FIR filter or as a IIRfilter.

U.S. Pat. No. 5,239,299 describes another ADC system comprising aplurality of individual converters, operated in time-interleaved mode.In this device, one converter is arbitrarily selected as referenceconverter, and the remaining converters are compensated for variationsin gain, offset, phase/frequency response by an array of FIR filters.

For a system with N ADCs and requiring a filter length of K for goodcorrection fidelity, the known solutions described above require N×Kdigital multipliers or, at best, (N−1)×K multipliers if one of the ADCsis selected as reference converter. Since digital multipliers areexpensive in terms of used silicon space and power consumption, afurther reduction of their number would be highly desirable.

In terms of silicon area, the space occupied by the filters of thedevices of the prior art may easily exceed that taken by the convertersthemselves. Space and cost considerations may, in some cases, interveneto limit the complexity and the depth of the filtering circuits adopted,and the precision of the correction must sometimes be sacrificed toachieve a simpler construction.

It is an aim of the present invention to provide an error-correctedinterleaved ADC, which is free from the above shortcomings of the knowndevices.

It is a further aim of the present invention to provide an interleavedADC, which requires a smaller number of components and can be producedin a more economical fashion, and which has a smaller size than theabove known devices.

It is a further aim of the present invention to provide an interleavedADC system exhibiting a more precise error correction, and a lowerdistortion of the digitized signal.

BRIEF SUMMARY OF THE INVENTION

These and other aims of the present invention are attained by thedevices comprising the features of the independent claims, with thedependent claims illustrating other optional features of the invention.In particular, these aims are attained by a Analogue to Digitalconverter circuit comprising:

-   -   a plurality of individual ADCs, each of said individual ADCs        having an individual response,    -   a clock generating circuit for clocking said individual ADCs,    -   said individual ADCs and said clock generating circuit being        arranged to generate samples of the amplitude of an input signal        by said individual ADCs one at a time in a cyclical ADC        sequence,    -   a digital filter for correcting differences between said        individual responses,    -   said digital filter comprising computing means, for computing a        function of said samples by said individual ADCs, said function        depending on a coefficient table loaded into said filter,    -   characterized in that said digital filter comprises coefficient        loading means (17) for changing said coefficient table,        synchronously with a transition of a signal of said clock        generating circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by referring to thedetailed description, and illustrated by the drawings in which:

FIGS. 1 a and 1 b represent a digital error-correction filter of knowntype, suitable for an interleaved ADC system.

FIG. 2 represents an interleaved ADC according to an aspect of thepresent invention.

FIG. 3 represents the functioning of a digital error-correction filteraccording to an aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 schematically represents an interleaved ADC according to thepresent invention. The ADC 30 comprises a clock generator 50 having afrequency F, for synchronizing the various operations of the ADC. Theclock generator may include a local clock oscillator, not represented,or generate a clock signal in dependence on an external clock input, forexample if the ADC is an element of a multi-channel acquisition system.An analogue signal, whose amplitude must be digitized, is present at ananalogue input 45. Although not represented on this figure, it is to beunderstood that the device of the invention may also include a varietyof analogue signal conditioning means, like for example filters,anti-aliasing filters, voltage protection networks, attenuators,impedance matching circuits, amplifiers or the like, and that theanalogue signal may be a voltage signal as well as a current signal.

The analogue signal 45 is fed to the inputs of a plurality of Nindividual ADC 71 for transforming into a time series of digitalsamples. In FIG. 2, N is set equal to four, by way of example. Howeverany number of individual ADCs may be implemented.

Each individual ADC 71 operates synchronously to an individual sub-clocksignal 55, whose frequency F/N is a sub-multiple of order N of thefrequency F of the overall clock signal 51, and whose phase is adaptedfor providing a regular interleaving of the samples generated by theindividual ADCs 71. A sub-clock generator 53 produces the requiredsub-clock signals, for example by an array of modulo-N dividers whoseinput is connected to the main clock signal 51.

The digital samples produced by the individual ADC 71 are then fed tothe digital filter 20. According to the circumstances, the correctedsignals are then stored in a memory buffer (not represented), for lateranalysis, or further processed on line.

FIG. 1 a represents a digital filter for an N ADC system of known type.This filter comprises N independent FIR blocks 12, each of whichcomputes a corrected signal for a specific ADC and operates at a clockfrequency F/N, synchronously with the corresponding ADC. The individualcorrection signals are then reunited by the multiplexer 80 into anoverall corrected signal.

Each of the FIR blocks of FIG. 1 a comprises an array of registers anddigital multipliers, as shown on FIG. 1 b.

The FIR block generates a linear combination of a series of successivesamples in the input signal, determined by a table of coefficients whosevalues are set in order to minimize the differences of the responses ofthe individual ADCs 71. In the represented architecture, each element ofthe combination corresponds to a register 41 and to a digital multiplier43.

The number of samples entering in the filter response may vary,according to the case. In the example of FIG. 3, only the five firstpairs of registers and multipliers are represented. It is to beunderstood, however that the present invention is not limited to aspecific number of samples. Each of the multipliers 43 multiplies thesample codes progressing along the FIR by a fixed coefficient A₁, A₂,etc. stored in a coefficient register 49. The coefficients are chosen inorder to optimally compensate the response differences of the Nindividual ADCs 71.

Although the individual FIR blocks 12 operate at the reduced clockfrequency F/N, each of the individual FIR blocks must process samplesfrom all the individual ADC's 71. This requires a large number ofmultipliers in the system, which results in an increased circuit sizeand cost.

According to an aspect of the present invention, explained now withreference to FIG. 3, the filter 20 comprises a FIR filter block 120.Advantageously, the FIR filter block 120 does not operate on the fulldynamics of the signal, but only produces a correction signal,representing the difference between the uncorrected codes 46 produced bythe ADC 71, and an “ideal” signal in which the ADCs 71 have identicalresponses. To this effect, the filter 20 comprises a digital delay 87,for storing the uncorrected signal codes during the computation of theerror signal by the FIR block 120, and a digital subtraction node 88 forcombining the correction signal and the uncorrected signal codes 46 intoa corrected signal 48.

Because the errors are generally small compared to the original signal,any inaccuracies in the correction system, caused by such things asmiscalibrations or rounding errors in the digital arithmetic, arereduced to “second order” errors, and thus this technique can be appliedto higher resolution acquisition systems than previously possible.

The FIR block 120 accepts as input the uncorrected multiplexed data 46coming from the N ADC 71, and is clocked at the full clock speed F. Eachof the coefficient registers 49 of FIG. 1 b is however replaced by abank 149 of N registers, thus defining N distinct coefficient tables A₁,A₂, A₃ . . . ; B₁, B₂, B₃ . . . ; C₁, C₂, C₃ . . . ; D₁, D₂, D₃ . . . ;one for each of the N ADCs.

A modulo-N counter 170 drives the multiplexers 177 so that thecoefficient table is reloaded between multiply operations. In this case,we have only one FIR block 120, which cyclically loads the coefficienttable for first ADC on data samples corresponding to first ADC, thecoefficient table for second ADC on data samples corresponding to secondADC, and so on.

By the disposition of the invention, one FIR block 120 replaces the Nblocks 12 of the known filter of FIG. 1 a. The number of requiredmultipliers is then equal to the length K of the FIR block, for anynumber of ADCs in the system. The number of multipliers is thus reducedby a factor N, which results in a space and power saving in the circuit.

The present invention comprises also the case in which the FIR filtersare replaced by IIR filters or by other kind of digital processors,producing a generic predetermined function of the uncorrected samples,in dependence on a coefficient table, and in which the coefficient tableis reloaded synchronously with the interleaving scheme of the individualADCs.

1. Analogue to Digital converter circuit comprising, a plurality ofindividual ADCs, each of said individual ADCs having an individualresponse, a clock generating circuit, for clocking said individual ADCs,said individual ADCs and said clock generating circuit being arranged togenerate samples of the amplitude of a input signal by said individualADCs one at a time in an cyclical ADC sequence, a digital filter forcorrecting differences between said individual responses, said digitalfilter comprising computing means, for computing a function of saidsamples by said individual ADCs, said function depending on acoefficient table loaded into said filter, characterized in that saiddigital filter comprises coefficient loading means for changing saidcoefficient table, synchronously with a transition of a signal of saidclock generating circuit.
 2. The Analogue to Digital converter of claim1, further comprising registers for storing individual coefficienttables relative to said individual ADCs, and wherein said coefficientloading means are operatively arranged to load said individualcoefficient tables one at a time in a cyclical sequence.
 3. The Analogueto Digital converter of claim 2, wherein said coefficient loading meansare operatively arranged to load said individual coefficient tablessynchronously with said cyclical ADC sequence.
 4. The Analogue toDigital converter of claim 3, containing one of said individualcoefficient tables for each of said individual ADCs.
 5. The Analogue toDigital converter of claim 1, wherein said digital filter comprises atleast one FIR filter.
 6. The Analogue to Digital converter of claim 1,wherein said computing means comprise an array of multipliers forextracting a linear combination of a time sequence of said samples. 7.The Analogue to Digital converter of claim 2, wherein said coefficientloading means comprise an array of multiplexers and a counter forcyclically loading one of said individual coefficient tables.
 8. Digitalprocessing circuit, for correcting digital signals produced by aplurality of interleaved individual digital sources, said digital filtercomprising computing means, for computing a function of digital samplesby said individual digital sources, said function depending on acoefficient table loaded into said filter, characterized in that saiddigital filter comprises coefficient loading means for changing saidcoefficient table, synchronously with the interleaving scheme of saidindividual digital sources.
 9. The Digital processing circuit of claim8, further comprising registers for storing individual coefficienttables relative to said individual sources, and wherein said coefficientloading means are operatively arranged to load said individualcoefficient tables one at a time in a cyclical sequence.
 10. The Digitalprocessing circuit of claim 8, wherein said interleaved digital sourcesare operatively arranged to generate a time series of digital samples,and said computing means comprise an array of multipliers for extractinga linear combination of a sequence of said samples.
 11. The Digitalprocessing circuit of claim 9, wherein said coefficient loading meanscomprise an array of multiplexers and a counter for cyclically loadingone of said individual coefficient tables.